The present invention relates to medical shock wave treatment and here particularly to the contactless comminution of concrements in the body of a living being under utilization of a submerged arc discharge serving as a shock wave generator, there being an appropriate switching and operating structure for the generator.
Concerning the technology generally and the comminution of concrements in particular, reference is made to German patent 23 51 247 corresponding to U.S. Pat. No. 3,942,531, and other patents of common assignee. Typical arc discharge devices and units in conjunction with which the invention can be practiced with advantage are shown e.g. in U.S. Pat. Nos. 4,608,983 and 4,809,682. Positioning electrode exchange units are shown e.g. in U.S. Pat. No. 4,040,050. Moreover, specific reference is made to a brochure by Ch. Chaussy ed., "Extracorporeal Shock wave Lithotripsy", giving an extensive background treatment on the subject mater including the comminution process itself and the generation of shock waves in particular.
Broadly speaking therapeutic shock wave generation uses a spark gap produced by a discharge between electrodes the discharge through the gap between the electrodes results from the discharge of an electric capacitor. That capacitor is discharged in certain instants. The capacitor is recharged thereafter, requiring a certain period of time for such a restoration but the discharge itself is a phenomenon of very short duration. Since the electrodes are submerged, on discharge a shock wave is produced in the water. A rotational ellipsoid partially encloses the spark gap which is situated in one of the two focal points of that rotational ellipsoid. As shock waves are produced by the gap in that one focal point they are reflected by the rotational ellipsoid and are refocused in the second focal point for comminuting concrement thereat.
The acoustic energy that can be concentrated in the body of the person, depends on the efficiency of coupling the waves to the body. From a primary point of view the discharge depends on the amount of electrical energy that is fed into the discharge device. Specific relevant parameters include the voltage and the capacitance of the capacitor. Generally speaking, one needs some form of control over the intensity and energy of the shock wave and, therefore, it is desirable to provide for a certain, possibly even large variability of the shock wave energy. Depending on the medical specifics; a rather broad energy spectrum is quite desirable.
Gall stones require usually a higher energy level for their comminution than is necessary for the destruction of kidney stones. It is obvious that the same equipment should be suited for both kinds of treatment, comminution of kidney stones, as well as comminution of gall stones. Consequently this requirement of multiple use carries with it the requirement of making available a fairly wide range of energy.
Shock wave energies can easily be varied through variation of the voltage that is applied to the capacitor for charging it. Variation of that voltage controls the amount of charge on the capacitor. However, the ignition properties of the submerged arc limit the range of variability of the voltage that can be accommodated. Another factor to be considered is that multiple use of the same electrodes entails certain burn-off which inherently increases the voltage minimum for obtaining any ignition at all. The required energy thus increases with the frequency of use and burn-off of the electrodes. Hence, the discharge and shock producing energy cannot be reduced to any level below that minimal level needed for the ignition process. Another way of controlling the shock wave energy, of course, is the variation of the capacitance. While such an approach is feasible in principle it requires high voltage and/or high current switches which are mechanically bulky and expensive.